1) Field of the Invention
The present invention relates to a document reading lens, a document reading lens unit, a document reader, and an image forming apparatus.
2) Description of the Related Art
A digital copier or a facsimile machine reads a document and converts an image of the document thus read to a signal. It is a document reading lens that forms the image of the document on a light reception unit of an imaging unit such as a charge coupled device (CCD).
As the document reading lens, a Gauss-type lens that includes six lenses in four groups is widely used. While the Gauss-type lens suppresses occurrence of a comatic flare even if an aperture of the lens is large, most of Gauss-type lenses have a half angle of view of about 18 degrees.
The Gauss-type lenses having relatively wide angles of view are disclosed in, for example, Japanese Patent Application Laid-Open No. H6-342120, Japanese Patent Application Laid-Open No. H8-146292, Japanese Patent Application Laid-Open No. H9-113802, Japanese Patent Application Laid-Open No. H9-171136, and Japanese Patent Application Laid-Open No. H9-304696. The widest angle of view of these lenses is about 20 degrees.
As a document reading lens of a type other than the Gauss-type, there is known a modified Xenor-type lens that includes six lenses in four groups similarly to the Gauss-type lens, for example, see Japanese Patent Application Laid-Open No. H10-333029. However, the disclosed lens has a half angle of view up to 17.6 degrees at the most.
If the angle of view of the document reading lens is narrow, an optical path length from the document to the imaging unit cannot be reduced and a reduction in the size of a document reader is difficult to achieve.